1. Field of the Invention
The present invention relates generally to a Liquid-Crystal Display (LCD) device. More particularly, the invention relates to an active-matrix addressing LCD device using lateral electric field, which improves the transmittance and the fabrication yield.
2. Description of the Related Art
Active-matrix addressing LCD devices that use Thin-Film Transistors (TFTs) as the switching elements for respective pixels, which provide high-level image quality, have been extensively used as display devices for portable or note-book type computers. Recently, they have been used as monitoring devices of space-saving desktop computers as well.
Active-matrix addressing LCD devices are classified into two types. With the devices of the first type, the orientation of the molecular axis of liquid crystal, which is called the “director”, is rotated in a plane perpendicular to the pair of substrates, thereby displaying desired images. With the devices of the second type, the “director” is rotated in a plane parallel to the pair of substrates, thereby displaying desired images. A typical one of the first type LCD devices is of the Twisted Nematic (TN) mode. A typical one of the second type LCD devices is of the In-Plane Switching (IPS) mode, which may be called the “lateral electric field” mode because the liquid crystal molecules existing in the liquid crystal layer are rotated or driven by electric field generated to be approximately parallel to the pair of substrates.
The IPS mode LCD device has an advantage that the obtainable viewing angle is wider than that of the TN mode LCD devices and therefore, the device of this type has been often used for large-scale display devices. This is due to the following reason. Specifically, with the IPS mode LCD device, a viewer or user always sees the displayed images approximately along the short axis of liquid-crystal molecules even if he/she moves his/her point of view. Therefore, the “tilt angle” of the liquid-crystal molecules has no or very low viewing angle dependence, resulting in a wider viewing angle.
On the other hand, the IPS mode LCD device has a disadvantage that an obtainable aperture ratio is low and as a result, the transmittance is reduced. This is because the driving electrodes, which are made of opaque, conductive material for the scan lines or the data lines, are formed on one of the pair of substrates coupled with each other in such a way as to keep a liquid crystal layer therebetween. Thus, various improvements have been discussed and developed to raise the transmittance so far.
One of the improvements developed before is disclosed in the Japanese Patent No. 3123273 published on Oct. 27, 2000. This improvement or technique has the following features:
(i) The parts of the signal lines, which face the liquid crystal layer, are partially covered with a conductor. (ii) The conductor is electrically connected to the source electrodes or the common electrode for applying the electric field approximately parallel to the substrates to the liquid crystal layer.
Because of these features (i) and (ii), undesired electric field from the signal lines is shielded or blocked with the common electrode, thereby expanding the effective display area of each pixel. As a result, the aperture ratio of each pixel is increased and therefore, the light utilization efficiency is enhanced.
Moreover, the Japanese Non-Examined Patent Publication No. 9-73101 published on Mar. 18, 1997 discloses an improvement that transparent material is used for making the electrodes for driving the liquid crystal, thereby enhancing the light utilization efficiency.
By the way, the active-matrix addressing LCD device has the basic operation principle as follows. This principle is applicable regardless of the operation mode of the LCD device.
Specifically, desired electric charges are written into the dielectric liquid crystal layer by way of the TFTs as the switching elements, thereby controlling the orientation of the liquid crystal molecules existing in the liquid crystal layer with the use of the electric field generated by the electric charges thus written. Thus, the transmitting state of external light through the liquid crystal layer is controlled to thereby display images on the screen of the LCD device as desired.
It is ideal that the electric charges written (i.e., the electric field generated) are kept until new electric charges are written into the liquid crystal layer at a next timing (i.e., within one frame). However, the liquid crystal has dielectric constant anisotropy and thus, the liquid crystal molecules are rotated according to the electric field. This leads to reduction of the electric field generated, which will be termed the “dielectric relaxation” below. To suppress the electric field reduction due to the dielectric relaxation, “storage capacitors”, the capacitance of which has a specific ratio to the capacitance of the liquid crystal capacitors, are formed to increase the quantity of electric charges to be written when the TFTs are turned on. As a result, even if the dielectric relaxation occurs and the electric field is reduced, the electric charges written into the storage capacitors are dispersed in the liquid crystal capacitors to compensate the electric field reduction.
The storage capacitors have an effect of suppressing the pixel voltage reduction (which is generally termed the “feed through voltage ΔVp”) that occurs when the TFTs are transferred from the turn-on state to the turn-off state. Therefore, these storage capacitors are used as a measure against flickers too.
The cause of the “feed through” is the parasitic capacitance Cgs between the gate electrode of the TFT and the source electrode thereof. Specifically, when the TFT is turned on by the gate pulse signal, electric charge is written and stored in the liquid crystal capacitor (capacitance: Ccl) and the storage capacitor (capacitance: Csc) in each pixel. At the moment the TFT is turned off, the electric charge that has been stored in the liquid crystal capacitor and the storage capacitor is redistributed to the respective capacitors, resulting in the “feed through” phenomenon. Since the LCD device using the lateral electric field does not require the transparent electrode formed on the color filter substrate (i.e., the opposite substrate) of the TN mode LCD device, the lines of electric force generated from the pixel electrodes and the common electrode will penetrate the color layer provided on the opposite substrate. In other words, the feed through voltage ΔVp of the LCD device using the lateral electric field is expressed as a function of the color layer capacitor (capacitance: Ccolor). As a result, the feed through voltage ΔVp is given by the following equation (1).ΔVp=Cgs/(Cgs+Csc+Clc+Ccolor)×(Vgon−Vgoff)  (1)where Vgon and Vgoff are turn-on and turn-off gate voltages of the TFT, respectively.
As understood from the above explanation, to suppress or decrease the feed through voltage ΔVp, it is necessary for the IPS mode LCD device to increase the storage capacitance Csc.
The explanation presented below will be made for the IPS mode LCD device as a typical example of the LCD devices using lateral electric field. However, needless to say, it is applicable to any other mode of the LCD devices using lateral electric field.
The storage capacitors in the IPS mode LCD device are typically realized by forming an interlayer dielectric layer between the pixel electrodes and a metal or conductive layer kept at a fixed voltage by two methods, the “common storage” method and the “gate storage” method.
The “gate storage” method is a method to form the storage capacitor between the prior-stage scanning line and the corresponding pixel electrode. In this method, the storage capacitor between the prior-stage scanning line and the corresponding pixel electrode serves as a load of the corresponding scanning line signal and therefore, there are disadvantages that the corresponding gate line signal is likely to be delayed and that the panel transmittance within the panel plane is likely to be dispersed.
On the other hand, the “common storage” method is a method to form the storage capacitor between the common electrode and the pixel electrode. In the IPS mode LCD device, the comb-tooth-shaped common electrode is provided in each pixel and thus, the storage capacitor is easily formed by the common electrode and the pixel electrode. Moreover, since no load is given to the scanning line signal, the scanning signal is not likely to be delayed. Accordingly, the “common storage” method is preferably used for large-scale IPS mode LCD devices.
The common electrode lines and the data lines are usually made of opaque, conductive material when the LCD device is large-sized. The reason is as follows:
Specifically, the common electrode lines need to be formed by using a low-resistance wiring material (e.g., a single layer of Cr, Ti, Mo, W, or Al or a multilayer structure of those metals) to prevent the propagation delay of the common electrode voltage or potential. Since these electrode materials are opaque, the areas covered with the common electrode lines do not serve as apertures and thus, they give no contribution to transmission of light. Moreover, when the common electrode lines are formed by the same material as that of the scanning lines in the same process step of forming the scanning lines to avoid the increase of the necessary fabrication process steps of the TFTs, low-resistance, opaque, conductive material needs to be used to lower the wiring resistance of the scanning lines and the common electrode lines and to protect the back channel sections of the TFTs against external light. In this case also, the areas covered with the common electrode lines do not serve as apertures and thus, they give no contribution to transmission of light. Additionally, low-resistance, opaque wiring material needs to be used to lower the wiring resistance of the data lines.
Moreover, if the common electrodes are formed to cover the data lines in order to prevent the electric field generated by the data line signals from being applied to the liquid crystal layer by way of the apertures, the parasitic capacitance between the data lines and the common electrode increases. This makes it likely to delay the transmission of the data line signals. To prevent the delay of the data line signals, the increase of the parasitic capacitances between the data lines and the common electrode needs to be suppressed. This is realized by forming an interlayer dielectric layer with a low dielectric constant between the data lines and the common electrode that shields the data lines, or by forming a thick interlayer dielectric layer with a comparatively high dielectric constant between the data lines and the common electrode. As a result, the storage capacitor with a sufficiently large capacitance for stable displaying operation is unable to be formed between the level of the data lines and the level of the common electrode. Instead, this capacitor needs to be formed between the level of the common electrode lines and the level of the data lines. If so, the interlayer dielectric layer between the common electrode lines and the data lines may be thinned to increase the capacitance of the said storage capacitor. However, the probability that the fabrication yield degrades due to the electrical short circuit between the lines will increase and at the same time, the switching characteristics of the TFTs will be badly affected. Accordingly, it is most effective for the TFT array that two common electrode lines are formed to sandwich the scanning line to increase the area of the storage capacitor.
Furthermore, with the IPS mode LCD device, as shown in FIG. 22, the direction of the electric field applied to the liquid crystal layer is complicated at the end of each “column”. The “column” is defined as an elongated area surrounded by the tooth of the comb-tooth-shaped common electrode and the adjoining tooth of the comb-tooth-shaped pixel electrode. Thus, the following phenomenon tends to occur due to the complicated electric field.
Specifically, a region (i.e., a normal domain) where the orientation of the liquid crystal molecules is rotated in a desired direction is formed and at the same time, another region (i.e., an abnormal domain) where the orientation of the liquid crystal molecules is rotated in an opposite direction to the desired direction is formed. In the abnormal domain, the orientation of the liquid crystal molecules is unable to be rotated in the desired direction unless the stronger electric field than that of the normal domain is applied to. As a result, the abnormal domain scarcely makes contribution to increase of the panel transmittance of the LCD device, which means that the panel transmittance is lowered. Moreover, since the orientation of the liquid crystal molecules is scarcely rotated at the boundary between the normal and abnormal domains regardless of the intensity of the applied electric field, the existence of the boundary lowers the panel transmittance in each pixel. Accordingly, some contrivance is necessary for the IPS mode LCD device to prevent the formation of the abnormal domain.
A technique for preventing the formation of the abnormal domain is disclosed by the Japanese Patent No. 2973934 published on Sep. 31, 1999. In this technique, the electrodes for driving the liquid crystal layer (i.e., the pixel electrodes and the common electrode) are formed to have the staggered or uneven patterns, where each of the electrodes has lateral protrusions and depressions. Using this staggered or uneven pattern of the electrodes, the electric field applied to the liquid crystal layer is controlled well.
As explained above, it is necessary to restrict or regulate the complicated electric-field direction at the column ends, and to sandwich the scanning line by two common electrode lines (in other words, to form two common electrode lines for each pixel). These are to prevent the disorder of the alignment of the liquid crystal molecules due to the leaked electric field from the scanning line signals and the opposite rotation of the liquid crystal molecules, thereby implementing desired LCD reliability improvement. Therefore, at least two storage capacitors for stabilizing the display operation can be formed for each pixel, thereby advantageously increasing the total storage capacitance, because two common electrode lines are provided for each pixel. However, if at least two capacitors are formed to be apart from each other in each pixel, patterned “pixel voltage or potential layers” need to be formed in the same level as the data lines and at the same time, the pixel potential layers need to be kept at the same potentials as those of the pixel electrodes applied through the TFTs.
If the pixel potential layers are formed in each pixel, which are used to form two or more storage capacitors in each pixel, are constituted in such a way as to be electrically connected to each other by way of parts of a conductive layer, it was found that the following problems occurred.
The first problem is that the total panel transmittance is lowered. Specifically, if a conductive layer for interconnecting the storage capacitors in each pixel with each other is formed by the same metal layer as the data lines and at the same time, the said metal layer is overlapped with the pixel electrodes, the rotation of the liquid crystal molecules caused by the applied electric field makes no contribution to the panel transmittance in the overlapping areas of the said metal layer and the pixel electrodes. As a result, the obtainable total panel transmittance is lowered.
The second problem is that the effective aperture ratio is decreased and the transmittance is lowered. Specifically, the conductive layer (i.e., the metal layer made of the same material as that of the data lines) and the pixel electrodes are formed in different levels in different process steps. Therefore, if an overlay error is present for these two layers, the overlapping areas of the conductive layer and the pixel electrodes expand and thus, the effective aperture ratio for each pixel decreases. This means that the transmittance is lowered.
The third problem is that the luminance is lowered in the all-white displaying operation. Specifically, in the overlapping areas of the conductive layer (i.e., the metal layer made of the same material as that of the data lines) and the comb-tooth-shaped pixel electrodes, the electric field strength is increased locally. Thus, the electric field fluctuates in each pixel, which results in the luminance lowering in the all-white displaying operation.
The fourth problem is that the fabrication yield is lowered. Specifically, the transparent pixel electrodes and the transparent common electrode tend to be disconnected locally due to the step-shaped gaps between the pixel and common electrodes and their underlying metal layers (i.e., the scanning lines and the data lines) in the etching process, resulting in undesired disconnections in the patterned electrodes. Thus, the lateral electric field is not applied partially to the liquid crystal layer, resulting in a defect in the displaying operation. This leads to lowering in the fabrication yield of the LCD device.